This application is a resubmission in response to PAR-03-056 and particularly to Research topic 3. Druq Discovery for Coqnitive Decline and Alzheimer's Disease. Cholesterol (CL) is considered one of the main players in the pathogenesis of Alzheimer's disease (AD). It has been shown that accumulation of excess CL in hippocampal neurons increased Abeta production. Furthermore, prevalence of AD is reduced among people taking statins - CL lowering drugs. In contrast, plasma levels of HDL-cholesterol (HDL-CL) are inversely associated with risk of cardiovascular disease and AD. Understanding the mechanisms through which CL homeostasis affects APP processing and amyloid deposition will provide clues for better understanding the risk factors, prevention and treatment of AD. Liver X receptors (LXRalpha and beta) act as molecular sensors of cholesterol levels and respond by inducing processes that reduce cholesterol levels. The ABCA1 transporter which is under the transcriptional control of LXRs, mediates CL efflux and secretion of excess CL from cells to lipid-flee Apo-lipoproteins. It is considered a major determinant of plasma HDL concentration. We found that LXR/RXR agonists treatment of primary neurons increased ABCA1 expression and CL effiux to apolipoproteins A-I and E3, consequently decreasing CL content in these cells. More importantly, we demonstrated that these ligands alone or in combination with apolipoprotein A-I caused a substantial reduction in the stability of APP Cterminal fragments and decreased Abeta production. We hypothesize that transcriptional upregulation of ABCA1 triggered by pharmacological activation of LXR will affect the amyloidogenic processing of APP, with a decreased Abeta secretion and ultimately decreased amyloid deposition in the brain of AD model animals. To test our hypothesis we propose: Aim 1. To examine the role of LXR ligands and apolipoprotein-mediated cholesterol efllux on APP intracellular transport and Abeta generation. In cells stably expressing APPsw we will determine if LXR ligand treatment with or without apolipoproteins affects APP and BACE 1 vesicular transport from Golgi to plasma membrane and their endocytosis. To determine if the effect of LXR in neurons requires transcriptional upregulation of endogenous ABCA1, we will apply the same LXR ligand treatments in primary neuronal cultures derived from ABCA1wt and ABCA1-/- mice. Aim 2. To determine the effect of LXR synthetic pharmacological ligands on Aa generation and amyloid deposition in APP transgenic mice. By using APP23 mice we will test the critical question whether in-vivo administration of synthetic LXR agonists T0901317 and Hypocholamide, and the transcriptional activation of ABCA1 modulates Aa generation/secretion or the amount of Abeta deposits in the brain of AD transgenic model. The results of our study will advance the understanding how genes and proteins controlling intracellular CL content and its redistribution in the brain influence hAPP processing and Abeta deposition. We believe that the pharmacological manipulation of these regulatory mechanisms will validate LXRs as a valuable molecular target in the drug development and discovery of new therapeutic agents for prevention or treatment of Alzheimer's disease.